There has been a known printing device with a print ribbon pivotally movable between a rest position and a print position. As shown in FIG. 1, for instance, a ribbon cassette holder 2 having a print ribbon cassette 1 mounted thereon is pivotally carried on a carriage frame 4 by means of a support shaft 6. The frame 4 is supported on a guide shaft 8 fixedly secured to a chassis (not shown) to be movable axially on the shaft 8 in a direction parallel to a platen 3. There is a stepping motor 10 movable forward and reverse mounted on the frame 4, whose rotary shaft bears a pinion gear 12 with the number of teeth of Z1 splined to the shaft. In mesh with the pinion gear 12 is a larger gear 14 whose number of teeth is Z2. Gear 14 is carried by the frame 4 to be rotatable forward and in reverse. The larger gear 14 has integrally mounted thereon a smaller gear 16 whose number of teeth is Z3. In mesh with the smaller gear 16 is a toothed sector element 18 which is secured to the ribbon cassette holder 2 and whose number of teeth is Z4. The number of teeth Z2 of the larger gear 14 is greater than the number of teeth Z1 of the pinion gear 12, and the number of teeth Z4 of the toothed element 18 is greater than the number of the teeth Z3 of the smaller gear 16. A reduction gearing 20 consists of the pinion gear 12, the larger gear 14, the smaller gear 16 and the toothed element 18.
There is a contact element 22 provided under the ribbon cassette holder 2. On the frame 4 a stationary element 24 is formed opposed to the contact element 22. The stationary element 24 is brought into contact with the contact element 22 when the ribbon cassette holder 2 is pivotally swung in the direction indicated by the arrow A in FIG. 1.
In such a printing device, a predetermined number of drive pulses are given to the stepping motor 10 when the power is turned on. As a result, the stepping motor 10 is turned in the direction indicated by the arrow A to swing the ribbon cassette holder 2 in the direction indicated by the arrow A via the reduction gearing 20. The contact element 22 is thus brought into contact with the stationary element 24 to once bring the stepping motor 10 out of pace. A particular excitation phase of the stepping motor 10 is then excited. The stepping motor 10 is then rotated in the direction opposite to the arrow A by such excitation of a particular excitation phase until it stops at a stabilized position given by excitation of the particular excitation phase. The stop position of the motor is now assumed to be the original position, with reference to which all the subsequent controls take place. Therefore, a certain clearance C1 will be generated between the contact element 22 and the stationary element 24.
When printing, a given number of drive pulses are fed to the stepping motor 10 to turn it in the direction opposite to the arrow A. Accordingly, the ribbon cassette holder 2 is swung to position ribbon 1a to the print position indicated by a double-dot-dash line in FIG. 1, thereby enabling printing.
However, such a conventional printing device has been associated with a problem in that if the stepping motor 10 is made smaller or the reduction gearing 20 should provide a greater gear ratio, the original position being detected is displaced by an angle corresponding to an angle .theta. of the certain excitation phase of the stepping motor 10.
In particular, a nominal design value of the clearance C1 when detecting the original position is calculated by the following equation and is usually a small value on the order of 0.1 or 0.2 mm. EQU C1=.theta..times.(Z1/Z2).times.(Z3/Z4).times.L1.times.K (1)
where .theta. is the angle of the exciting phase of the stepping motor 10, L1 is a distance between the center of the support shaft 6 and the contact element 22, and K is a predetermined safety factor constant.
When the contact element 22 is brought into contact with the stationary element 24, for instance, in detecting the original position, the amount of rebound may be larger than the clearance C1. This results in that the original position of the stepping motor 10 may be displaced through the angle .theta. of the excitation phase, whereby the print position is also displaced through an angle corresponding to the angle .theta. of the excitation phase. To avoid this, it has been necessary to design the assembly to maintain the rebound within the clearance C1. This however contradicts the requirement to make the stepping motor 10 smaller and to provide a greater reduction gear ratio of the reduction gearing because the clearance C1 would then be reduced to an undesirable value, resulting in difficulties in assembly and adjustment and therefore in a non-uniform printing quality.